--- Comment #6 from Jonathan Wakely <redi at gcc dot> ---
I think you misunderstand. Listing all the individual -fxxx options without -O1
results in NO OPTIMIZATION. The difference you see is due to all the passes
enabled by -O1, not by the ones without flags.

As I wrote on stackoverflow:

If you don't use one of the -O1, -O2, -O3, -Ofast, or -Og optimization options
(and not -O0) then no optimization happens at all, so adjusting which
optimization passes are active doesn't do anything.

To find which optimization pass makes the difference you can turn on -O1 and
then disable individual optimization passes until you find the one that makes a

i.e. instead of gcc -fxxx -fyyy -fzzz ... use gcc -O1 -fno-xxx -fno-yyy

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